Control of the photoreactivity of diarylethene derivatives by quaternarization of the pyridylethynyl group

Article abstract of DOI:10.1021/ol8005216

Photochromic behavior of diarylethene derivatives with (4-pyridyl)ethynyl group directly attached to the 6-.π hexatrlene moieties of the diarylethenes was investigated. Upon quaternarization of the pyridine moieties, the photoreactivity was strongly suppr

Full text of DOI:10.1021/ol8005216

ORGANIC
LETTERS
2008
Vol. 10, No. 10
2051-2054
Control of the Photoreactivity of
Diarylethene Derivatives by
Quaternarization of the Pyridylethynyl
Group
Koji Yumoto,† Masahiro Irie,‡ and Kenji Matsuda*,†
Department of Chemistry and Biochemistry, Graduate School of Engineering, Kyushu
UniVersity, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan, and Department of
Chemistry, Rikkyo UniVersity, 3-34-1 Nishi-Ikebukuro, Toshima-ku,
Tokyo 171-8501, Japan
kmatsuda@cstf.kyushu-u.ac.jp
Received March 7, 2008
ABSTRACT
Photochromic behavior of diarylethene derivatives with (4-pyridyl)ethynyl group directly attached to the 6-π hexatriene moieties of the
diarylethenes was investigated. Upon quaternarization of the pyridine moieties, the photoreactivity was strongly suppressed. On the other
hand, diarylethene derivatives with nonconjugated (4-pyridyl)ethyl group exhibited the photochromic reactivity, regardless of whether pyridyl
rings are quaternarized or not. In the case of the (4-pyridyl)ethynyl-substituted compounds, the photochromic reactivity was suppressed by
the addition of trifluoroacetic acid and was restored by diethylamine.
For several decades, much attention has been focused on
the compounds that undergo photoinduced switching of their
properties, such as geometrical structure, redox potential,
magnetic interaction, and electric conductivity.1 Diaryl-
ethenes undergo reversible photoinduced cyclization/cyclo-
reversion reactions upon irradiation with UV and visible light
and have an advantage over other photochromic compounds
in the sense of thermal stability and fatigue resistance.2 The
photochromic reactivity depends on several factors, such as
(1) (a) Molecular Switches; Feringa, B. L., Ed.; Wiley-VCH: Weinheim,
Germany, 2001. (b) Raymo, F. M.; Tomasulo, M. Chem. Soc. ReV. 2005,
34, 327–336. (c) Gust, D.; Moore, T. A.; Moore, A. L. Chem. Commun.
2006, 1169–1178.
† Kyushu University.
‡ Rikkyo University.
10.1021/ol8005216 CCC: $40.75
Published on Web 04/22/2008
2008 American Chemical Society

conformation of the open-ring isomer, electron donor/
acceptor substituents, and conjugation length of the heteroaryl
groups.3 Particularly, substituents on the reactive position
greatly influence the photoreactivity. The introduction of
methoxy groups to the reactive positions renders the ring-
opening reaction quantum yield extraordinarily small.4 It is
worth noting that the orbital hybridization of the reactive
carbon is interconverted from sp2 to sp3 accompanied by the
photocyclization.5
Scheme 1. Synthesis of Diarylethenes 1a-4a
When the nature of the substituents at the reactive positions
is controlled by external stimulus, the photoreactivity is also
anticipated to be controlled. The control of the photoreactivity
by external stimuli such as pH or electric potential is
important for the practical application of diarylethenes,
because a controlled photoreactivity, namely a gated reactiv-
ity, can provide a nondestructive readout capability.6 Pyridyl
group is one of the external stimuli-responsive substituents
that are quaternarized by alkylating reagent or protic acid.
According to this idea, we have synthesized diarylethene
derivatives having (4-pyridyl)ethynyl groups and (4-pyridyl)-
ethyl groups at the reactive positions and studied the control
of the photochromic reactivity by the quaternarization of the
nitrogen atoms on the pyridine rings.
We have synthesized dithienylethenes 1-4 with pyridyl
or N-methylpyridinium substituents at the 2-positions of the
thiophene rings (Figure 1). Diarylethene 1 and its N-meth-
mass spectroscopies and elemental analysis. X-ray crystal-
lographic analysis of 1a, 2a, and 4a were also carried out.8
Figure 2 shows the photochromic absorption spectral
change of compound 1 in dichloromethane. Upon irradiation
with 313 nm light, a new band appeared in the visible region.
The solution turned blue, which suggests the formation of
the closed-ring isomer 1b. The absorption maximum of 1b
Figure 1
groups.
. Diarylethenes with (4-pyridyl)ethynyl and (4-pyridyl)ethyl
(2) (a) Irie, M. Chem. ReV. 2000, 100, 1685–1716. (b) Matsuda, K.;
Irie, M. J. Photochem. Photobiol. C 2004, 5, 169–182. (c) Tian, H.; Wang,
S. Chem. Commun. 2007, 781–792.
ylated derivative 2 have an elongated π-electron system on
the reactive position. Meanwhile, compounds 3 and 4 have
no π-conjugation at the reactive position because of the
single-bond linker.
(3) (a) Takeshita, M.; Kato, N.; Kawauchi, S.; Imase, T.; Watanabe, J.;
Irie, M. J. Org. Chem. 1998, 63, 9306–9313. (b) Irie, M.; Sakemura, K.;
Okinaka, M.; Uchida, K. J. Org. Chem. 1995, 60, 8305–8309. (c) Irie, M.;
Eriguchi, T.; Takada, T.; Uchida, K. Tetrahedron Lett. 1997, 53, 12263–
12271.
The synthesis was performed according to Scheme 1. 2,4-
Dibromobenzene (5) was converted to 2,3-dibromo-5-phen-
ylthiophene (7) by the literature method.7 Introduction of
(4-pyridyl)ethynyl group was carried out with Sonogashira-
Hagihara coupling to give 8, which is then converted to
diarylethene 1. Pd/C-catalyzed hydrogenation of diarylethene
1 was performed to give (4-pyridyl)ethyl derivative 3. The
preparation of quaternarized derivatives 2a and 4a was
conducted by N-methylation of 1a and 3a with iodomethane
(4) Shibata, K.; Kobatake, S.; Irie, M. Chem. Lett. 2001, 618–619.
(5) (a) Tanifuji, N.; Irie, M.; Matsuda, K. J. Am. Chem. Soc. 2005, 127,
13344–13353. (b) Odo, Y.; Matsuda, K.; Irie, M. Chem.-Eur. J. 2006, 12,
4283–4288.
(6) (a) Yokoyama, Y.; Yamane, T.; Kurita, Y. J. Chem. Soc., Chem.
Commun. 1991, 1722–1724. (b) Irie, M.; Miyatake, O.; Uchida, K. J. Am.
Chem. Soc. 1992, 114, 8715–8716. (c) Matsui, F.; Taniguchi, H.; Yokoyama,
Y.; Sugiyama, K.; Kurita, Y. Chem. Lett. 1994, 1869–1872. (d) Kawai,
S. H.; Gilat, S. L.; Ponsinet, R.; Lehn, J.-M. Chem.-Eur. J. 1995, 1, 285–
293. (e) Uchida, K.; Matsuoka, T.; Kobatake, S.; Yamaguchi, T.; Irie, M.
Tetrahedron 2001, 57, 4559–4565.
(7) Morimitsu, K.; Kobatake, S.; Irie, M. Tetrahedron Lett. 2004, 45,
1155–1158.
and subsequent counterion exchange from I- to PF6 . The
-
structures of the compounds were confirmed by NMR and
(8) See Supporting Information.
2052
Org. Lett., Vol. 10, No. 10, 2008

Table 1. Absorption Characteristics and Photoreactivity of 1 and
3 in CH2Cl2, 2 and 4 in CH3CN, and 9 in Hexane
a
b
compd ꢀopen /M-1cm-1 ꢀclosed /M-1cm-1 ΦOC ΦCO convn /%c
1
2
3
4
44400 (309 nm) 15300 (581 nm) 0.14 0.27
40800 (295 nm) N/A N/A N/A
58
0
>99
>99
97
37200 (289 nm) 14300 (613 nm) 0.45 0.050
35700 (289 nm) 12500 (614 nm) 0.47 0.082
912 35600 (280 nm) 15600 (575 nm) 0.59 0.013
a Cyclization quantum yields were measured using 313 nm light.
b Cycloreversion quantum yields were measured using 517 nm light.
c Conversion under irradiation with 313 nm light.
Figure 2. (a) Photochromic spectral change of 1 in CH2Cl2. Solid
line: open-ring isomer 1a; dashed line: closed ring isomer 1b; dotted
line: in the photostationary state under irradiation with 313 nm light.
(b) Absorption spectrum of 2a in CH3CN. 2a is photoinactive.
pyridine rings, but when the linker is a nonconjugated single
bond, the compounds show the photochromic reactivity
regardless of whether the pyridine rings are quaternarized
or not.
was observed at 581 nm. The cycloreversion of 1b back to
the open-ring isomer occurred by irradiation with visible light
(λ > 480 nm).
The quantum yields were measured in CH2Cl2 for 1 and
3 and in CH3CN for 4.10 As references, furylfulgide and 1,2-
bis(2-methylbenzothiophen-3-yl)perfluorocyclopentene were
used for the cyclization and the cycloreversion reactions,
respectively.11 The measured quantum yields are summarized
in Table 1. The cyclization quantum yields of diarylethenes
3 and 4, which have single-bond linker, are more than 3 times
larger than that of diarylethene 1, which has triple-bond
linker. The cycloreversion quantum yields of 3 and 4 are
much smaller than that of 1. As a consequence of the change
in the quantum yields, high conversion from the open- to
the closed-ring isomer under irradiation with UV light is
achieved for the diarylethenes 3 and 4. There is almost no
difference in photochromic properties between 3 and 4. Both
the cyclization and cycloreversion quantum yields of 3 and
4 are similar with the quantum yield of the parent compound
1,2-bis(2-methyl-5-phenyl-3-thienyl)perfluorocyclopentene
(9).12 The N-methylpyridinium cation substituent at the
reactive position strongly perturbs the 6-π hexatriene con-
jugation through the triple-bond linker, but the electronic
nature of the substituents is not transmitted through the
single-bond linker.
However, cationic species 2a did not show any spectral
change upon irradiation in any solvent. It is worth noting
that dithienylethene derivative containing N-methylpyri-
dinium substituents at 5-positions of the thiophene rings, is
reported to show reversible cyclization/cycloreversion.9 The
electron-withdrawing nature of the N-methylpyridinium
cation at the reactive position is considered to affect the 6-π
electron system through π-conjugation and to suppress the
photoreactivity.
To confirm the electronic effect of the pyridinium cation,
photochromic performance of 3 and 4, which have single-
bond linker between the pyridine ring and the hexatriene
moiety, was examined. The absorption spectral changes are
shown in Figure 3. Both 3 and 4 showed reversible
By adding protic acid, quaternarization of the nitrogen of
the pyridine ring can also be accomplished. Because the
photoreactivity of diarylethene 1 was effectively changed by
the quaternarization of the pyridine ring, the reactivity is also
anticipated to be controlled by the addition of acid and base.
Figure 4 shows the photochromic behavior of dichlo-
romethane solution of 1a after the addition of trifluoroacetic
acid (TFA) and diethylamine. After the addition of TFA,
the absorption edge of diarylethene 1a showed a bathochro-
mic shift, which is also observed for 2 (Figure 4b). As
expected, after addition of an excess amount of TFA a new
band did not appear in the visible region upon 313 nm light
Figure 3. Photochromic spectral change of (a) 3a in CH2Cl2 and
(b) 4a in CH3CN. Solid line: open-ring isomers 3a and 4a; dashed
line: closed ring isomer 3b; dotted line: in the photostationary state
under irradiation with 313 nm light. 4b could not be isolated.
photochromism by irradiation with UV and visible light. The
spectra of both compounds are found to be very similar. The
absorption maxima of the closed-ring isomers 3b and 4b are
red-shifted more than 30 nm in comparison with 1b. When
the linker is a conjugated triple bond, the photoreactivity is
suppressed by N-methylation of the nitrogen atoms on the
(10) In spite of our best efforts, we could not isolate closed-ring isomer
4b. However, we estimated conversion to be >99% by using 1H NMR. 1H
NMR change is shown in the Supporting Information.
(11) (a) Heller, H. G.; Langan, J. R. J. Chem. Soc., Perkin Trans 2 1981,
341–343. (b) Uchida, K.; Tsuchida, E.; Aoi, Y.; Nakamura, S.; Irie, M.
Chem. Lett. 1999, 28, 63–64.
(9) (a) Gilat, S. L.; Kawai, S. H.; Lehn, J.-M. Chem.-Eur. J. 1995, 1,
275–284. (b) Matsuda, K.; Shinkai, Y.; Yamaguchi, T.; Nomiyama, K.;
Isayama, M.; Irie, M. Chem. Lett. 2003, 32, 1178–1179.
(12) Irie, M.; Lifka, T.; Kobatake, S.; Kato, N. J. Am. Chem. Soc. 2000,
122, 4871–4876.
Org. Lett., Vol. 10, No. 10, 2008
2053

Figure 4. Control of the photochromic reactivity of 1a by the addition of acid and base in CH2Cl2. Solid line: open-ring isomer; dotted line:
in the photostationary state under irradiation with 313 nm light. (a) In CH2Cl2; (b) in CH2Cl2 in the presence of 10 equivalents of trifluoroacetic
acid; (c) after addition of 10 equivalents of diethylamine to (b).
2+
irradiation. The generated quaternary cation species 1aH2
did not undergo cyclization reaction. Then the sample was
neutralized by the addition of diethylamine. The spectral
shape became almost identical with original form 1a. After
the addition of base, the photochromic reactivity was restored
as shown in Figure 4c. The protonated closed-ring isomer
addition of acid, which is attributed to the fact that the cation
is isolated from the chromophore by sp3 carbon at the reactive
2+
position. Protonated 1bH2
was thermally stable and
2+
converted to 1aH2 by irradiation with visible light. The
addition of acid and base enabled switching the photoactive
and the photoinactive states, affording gated reactivity
(Scheme 2).
2+
1bH2 could be generated by adding acid to a solution of
isolated 1b.13 The absorption maximum did not shift upon
In conclusion, a series of diarylethene derivatives with
pyridyl rings at reactive positions of thiophene rings were
synthesized. Although the photoreactivity was strongly
suppressed upon quaternarization of the nitrogen atoms of
(4-pyridyl)ethynyl groups, the derivative with (4-pyridyl)-
ethyl groups showed photochromic reactivity regardless of
whether pyridyl rings are quaternarized or not. The photo-
chromic reactivity depends on the π-conjugation between
pyridinium cation and the 6-π hexatriene conjugation.
Scheme 2
. Schematic Representation of the Control of the
Photoreactivity by the Addition of Acid and Base
Acknowledgment. This work was supported by a Grant-
in-Aid for Science Research in a Priority Area “New
Frontiers in Photochromism” (471) (No. 19050009) from the
MEXT, Japan.
Supporting Information Available: Experimental meth-
ods, 1H NMR spectra of synthesized compounds, photochro-
mic NMR spectral change of 4, absorption spectral change
of 1b upon addition of acid, and X-ray crystallographic
analysis. CIF files of 1a, 2a, and 4a. This material is available
free of charge via the Internet at http://pubs.acs.org.
OL8005216
(13) See Supporting Information. λmax: 1aH22+, 304 nm; 1bH22+, 580
nm.
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Org. Lett., Vol. 10, No. 10, 2008